Images of earth-based objects are routinely captured by satellites in orbit around earth. These images are used in significant and often critical scientific, military and intelligence operations and studies. Typically, the orbiting satellites transmit captured images to earth-based stations for review and study. However, images of earth-based objects, received from a satellite transmission, are often distorted due to effects of the atmosphere. Light reflected from an earth-based object must travel through a thick layer of relatively dense atmosphere before being received by a sensor on board an orbiting satellite. The air in the atmosphere, as well as substances suspended in the atmosphere, such as water droplets and dust, can scatter light that is captured by a sensor on board the satellite. For example, light from the sun can illuminate a microscopic dust particle, that then reflects back to the satellite based sensor. The light measured at each sensor location, (and thus, each pixel of the image) includes the light reflected from the surface of the earth, and all of the light scattered towards the sensor from particles in the path between the earth surface and the orbiting satellite.
Additive intensity at a sensor, caused by light scattered back from particles in the atmosphere, is referred to as path radiance. Path radiance can occur whenever an imaged object is a significant distance from the sensor, and the medium between the object and the sensor is not a vacuum. The removal of path radiance effects is an important objective of designers of systems that involve capturing images from a large distance, such as the design of satellite transmission systems.
In addition to path radiance, similar effects can occur when an image is recorded through transparent glass, cellophane, or other transparent or semi-transparent materials. Other phenomena, such as, for example, ambient specular reflection, while functionally different from path radiance, have a similar effect of increasing measured intensity in different color bands beyond what would be expected in a simple color model such as lambertion reflection.